: Parkinson's disease (PD) is the second most prevalent neurodegenerative disease (NDD) in America, afflicts about 500,000 citizens, and is a source of worsening disability for the individual and a substantial cost to society. The cause of progressive death of dopamine neurons in PD midbrain is not known. Increasing evidence and an excellent animal model (systemic MPTP treatment) suggest that a major cause of PD is defective functioning of complex I in the mitochondrial electron transport chain (ETC). What is not known is how a generalized defect in ETC function results in loss of a specific neuronal population (i.e., what is the origin of selective vulnerability?) and what cellular mechanisms cause dopamine neuronal death. The experiments in this revised proposal utilize microdialysis and molecular techniques to address five Specific Aims that test three hypotheses about the causes of dopamine neuronal death. Aim 1 will define the time course of formation of brain regional nitrotyrosine and hydroxyl free radicals and striatal immediate early gene activation during MPTP toxicity, and will determine the dependencies of these processes upon glutamate and dopamine receptors. Aim 2 further characterizes the interactions of MPTP and MPP+ with brain nitric oxide synthase (NOS) , as a potential cause of regional vulnerability. Aim 3 explores the effect of MPTP toxicity upon changes in neurotrophin metabolism and will determine if alterations in expression of protective enzymes occur. Aim 4 defines the sequence of events leading to apoptotic death of nigral neurons and characterized the interactions and regulation of growth regulatory proteins p53, bcl-2/x and bax in striatum and substantia nigra. In Aim 4 the apparent dependence on p53 expression of MPTP neurotoxicity will be further defined. Aim 5 will establish a cell culture model (SY5Y human neuroblastoma) for studying oxygen free radical production, neurotrophin influences on MPP+ toxicity and regulation of p53, bcl-2/x and bax. The goal of these studies is to provide focus for similar examination of human postmortem PD tissues and ultimately, to improve neuroprotective strategies for treatment of the disease. The results of experiments in each Specific Aim have potential treatment implications and will provide knowledge applicable to other human neurodegenerative diseases.